How to Assert on a Log Message in JUnit?

How to Assert on a Log Message in JUnit

Introduction:

Testing the behavior of code that communicates with external systems often involves verifying the content and format of messages generated. In Java, logging is a common way to record such messages. This article explores how to perform unit testing on log entries using the JUnit framework.

JUnit Assertions on Log Messages:

The code snippet provided in the question aims to assert that a certain log entry with a specific level (e.g., INFO) and message ("x happened") was generated by the code under test. To achieve this, you can employ a custom logger or handler that intercepts and stores log events.

Custom Logger/Handler Approach:

This approach requires creating a specialized logger or handler that captures logging events. The log statements in the code under test are then directed to this custom logger, enabling the retrieval and inspection of logged messages. However, this approach can be complex to implement and requires modifying the code under test.

Log Collector Pattern:

A more convenient solution is to use a custom appender that records logging events in a collection or list. Here's how you can achieve this:

1. Create an Appender:
   Implement a custom Appender subclass, overriding the append() method to store logging events in a data structure (e.g., a list).
2. Add Appender to Logger:
   Add an instance of your custom appender to the desired logger. This step can be performed within the test method or using annotations like @Before and @After.
3. Logging Message Assertion:
   After executing the code under test, retrieve the list of logged events from your appender. You can then use JUnit's assertion methods to verify the level, message, and other attributes of the log entry, as demonstrated in the provided code snippet.

Sample Implementation:

The following sample code illustrates the log collector pattern:

<code class="java">import org.apache.log4j.AppenderSkeleton;
import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;

import static org.junit.Assert.assertThat;
import static org.hamcrest.CoreMatchers.is;

public class LogCollectorTest {

    @Test
    public void testLogMessage() {
        // Create a custom appender to collect logging events.
        final TestAppender appender = new TestAppender();

        // Add the appender to the logger we wish to test.
        final Logger logger = Logger.getLogger(LogCollectorTest.class);
        logger.addAppender(appender);

        // Execute code under test.
        logger.info("Test");

        // Retrieve the list of logged events.
        final List<LoggingEvent> logs = appender.getLog();

        // Perform assertions on the first log entry.
        assertThat(logs.get(0).getLevel(), is(Level.INFO));
        assertThat(logs.get(0).getMessage(), is("Test"));
    }

    private static class TestAppender extends AppenderSkeleton {
        private final List<LoggingEvent> log = new ArrayList<>();

        @Override
        public boolean requiresLayout() {
            return false;
        }

        @Override
        protected void append(LoggingEvent loggingEvent) {
            log.add(loggingEvent);
        }

        @Override
        public void close() { }

        public List<LoggingEvent> getLog() {
            return new ArrayList<>(log);
        }
    }
}</code>

Conclusion:

The log collector pattern provides a reusable and convenient way to assert on log messages in JUnit tests. By capturing logged events in a data structure, you can easily verify their content and format, ensuring the correctness of your code's interaction with logging systems.

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